In recent years, non-destructive sensing techniques using electromagnetic waves in a frequency range between 30 GHz and 30 THz (terahertz waves) have been developed.
As a terahertz-wave detecting method, a method that employs a nonlinear optic crystal is widely used. Typical examples of nonlinear optic crystals include LiNbOx (lithium niobate, referred to as “LN” hereinafter), LiTaOx, NbTaOx, KTP, DAST, ZnTe, and GaSe. For detecting a terahertz wave using a nonlinear crystal, a Pockels effect (which is a kind of a second-order nonlinear phenomenon), which is a first-order electro-optic effect, is used. Specifically, when light is irradiated as probe light onto the same location as a terahertz wave, the polarization state of the probe light changes in accordance with the electric field of the terahertz wave. The amount of change in the polarization state is detected by a polarizing element and a light detector (see PTL 1). In an element that uses such a nonlinear crystal, the wavelength band of the probe light is wide so as to allow for a 0.8-μm band or even a so-called communication wavelength band of 1 μm or greater, thereby advantageously allowing for the use of an inexpensive light source, such as a fiber laser.
In PTL 1, the polarization of the probe light is changed by so-called vertical operation. Since the thickness of the crystal is equivalent to the interaction distance, the sensitivity can be increased with increasing thickness by performing phase-matching. However, in order to achieve phase-matching with a terahertz wave in a wideband, the crystal needs to be reduced in thickness, meaning that the sensitivity and the frequency band are in a trade-off relationship. For improving sensitivity by increasing the interaction distance, there has been a proposal in which the nonlinear crystal is operated horizontally (see NPL 1). In this case, a Cerenkov phase-matching method that utilizes dispersion of the terahertz wave and the probe light within the nonlinear crystal is discussed as a phase-matching method.
There have also been proposals with regard to generating a terahertz wave by a Cerenkov phase-matching method (see PTL 2 and NPL 2).